SIGKILL

Question

程序fork_wait.c :

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

int main(){
    pid_t a = fork();
    if (a != 0) {
        int status;
        printf("I am your father. (...) You know it to be true !");
        wait(&status);
    }
    else {
        printf("NOOOOOOOOOOOOOOOOOOOOO!!!! NOOOOOOO!!");
        sleep(2000);
    }
    return 0;
}

Bash :

$ cc fork_wait.c -o fork_wait && ./fork_wait
I am your father. (...) You know it to be true !
NOOOOOOOOOOOOOOOOOOOOO!!!! NOOOOOOO!!

执行时Bash :

$ pgrep fork_wait
37818
37819

$ kill -9 $(pgrep fork_wait | tail -1)
$ pgrep fork_wait
(nothing)

当进程被SIGTERM杀死时，谁会发送SIGCHLD信号？如果我杀了儿子，为什么没有僵尸程序？

Answer 1

这是你第一个节目的改编本。它使用默认的SIGCHLD信号处理，这意味着当子进程死亡(forkwait29.c)时，信号不会(由内核)传递给父进程：

#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>

int main(void)
{
    pid_t a = fork();
    if (a != 0)
    {
        printf("%5d: I am your father. (...) You know it to be true!\n", getpid());
        int corpse;
        int status;
        while ((corpse = wait(&status)) > 0)
            printf("PID %5d exited with status 0x%.4X\n", corpse, status);
    }
    else
    {
        printf("%5d: NOOOOOOOOOOOOOOOOOOOOO!!!! NOOOOOOO!!\n", getpid());
        sleep(2000);
    }
    return 0;
}

一个示例运行结果：

$ forkwait29
64001: I am your father. (...) You know it to be true!
64002: NOOOOOOOOOOOOOOOOOOOOO!!!! NOOOOOOO!!
PID 64002 exited with status 0x0009
$

您可以分析返回的状态(使用WIFSIGNALED、WIFTERMSIG、WIFEXITED、WEXITSTATUS、WCOREDUMP等)，它将显示儿童死亡是因为它收到信号9，SIGKILL。正如评论中所指出的，您的父母收集了死子进程(防止它成为一个(持久的)僵尸)并退出。

您可以添加一些这样的信号处理(forkwait73.c)：

#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <sys/wait.h>

static volatile sig_atomic_t sig_caught = 0;

static void sig_handler(int signum)
{
    sig_caught = signum;
}

int main(void)
{
    pid_t a = fork();
    if (a != 0)
    {
        printf("%5d: I am your father. (...) You know it to be true!\n", getpid());
        struct sigaction sa = { 0 };
        sa.sa_handler = sig_handler;
        sa.sa_flags = SA_RESTART;
        if (sigemptyset(&sa.sa_mask) != 0)
            return 1;
        sigaction(SIGCHLD, &sa, NULL);
        int corpse;
        int status;
        while ((corpse = wait(&status)) > 0)
        {
            printf("PID %5d exited with status 0x%.4X (caught = %d)\n",
                   corpse, status, sig_caught);
        }
    }
    else
    {
        printf("%5d: NOOOOOOOOOOOOOOOOOOOOO!!!! NOOOOOOO!!\n", getpid());
        sleep(2000);
    }
    return 0;
}

制作了一个样本运行：

$ forkwait73
63964: I am your father. (...) You know it to be true!
63965: NOOOOOOOOOOOOOOOOOOOOO!!!! NOOOOOOO!!
PID 63965 exited with status 0x000F (caught = 20)
$

当我在SA_RESTART Moneterey 12.2.1上省略了macOS标志时，得到的结果如下：

$ forkwait73
63929: I am your father. (...) You know it to be true!
63930: NOOOOOOOOOOOOOOOOOOOOO!!!! NOOOOOOO!!
$

如果没有SA_RESTART，父进程不会报告其子进程的死亡，因为wait()在errno == EINTR中失败了。与其设置SA_RESTART，不如将循环修改为：

        while ((corpse = wait(&status)) > 0 || errno == EINTR)
        {
            printf("PID %5d exited with status 0x%.4X (caught = %d)\n",
                   corpse, status, sig_caught);
            errno = 0;
        }

如果您希望看到僵尸进程，则必须安排父进程暂时不为子进程wait()。您也可以让它休眠，当sleep()调用结束时，它可以继续使用wait()循环。你也可以安排它去处理另一个信号，这样你就可以让它醒过来，发现它的孩子已经死了。